This invention relates to methods for measuring acoustic wave travel times in earth formations in the vicinity of a well borehole. More particularly, the present invention relates to techniques for measuring multiple acoustic wave component or wave propagation modes travel times in earth formations in the vicinity of a well borehole using swept frequency transmitting techniques.
Acoustic or sonic well logging has become an important method for determining the physical characteristics of earth formations in the vicinity of a well borehole. Measurements of the acoustic compressional wave velocity, or travel time, between a transmitter and receiver in a well borehole can define physical characteristics of the earth formations which are indicative of the capability of these formations to produce oil or gas. For example, a measurement of the compressional wave travel time gives a direct indication of the porosity of the formation in the vicinity of the well borehole. Such velocity or travel time measurements have therefore become practically standard for all new wells which are drilled.
In the prior art, acoustic or sonic well logging techniques have been used to measure the travel time or velocity of acoustic waves in the formations in the vicinity of the borehole. These methods typically use pulsed acoustic transmitters. An acoustic transmitter is fired impulsively, or pulsed, and the length of time necessary for the acoustic wave pulse generated by the transmitter to propagate from the transmitter through the earth formation to a receiver located a spaced distance away from the transmitter is measured. By appropriately combining the measurements of acoustic wave travel time at several receivers spaced different distances from either a single or multiple acoustic transmitters, then the acoustic wave travel time or sonic velocity of propagation of the formation may be determined. Quite elaborate schemes and geometrical considerations for eliminating the effect on the travel time measurement of the borehole and borehole fluids have also been developed.
In more recent years, it has become desired to measure other acoustic wave mode travel times than merely the compressional wave velocity. For example, in U.S. Pat. No. 4,131,875 issued Dec. 26, 1978 techniques are described for measuring so called "late arrival" waves or Stonely waves. Similarly, other prior art techniques such as that described in U.S. Pat. No. 3,354,983 issued Nov. 28, 1967, describes techniques for measuring acoustic shear wave velocities. In all of these techniques, an acoustic pulse is generated by the transmitter and the wave form of the acoustic signal at one or more receivers is analyzed, in order to determine the velocity of either compressional, shear or Stonely waves in the vicinity of the borehole.
Pulsed acoustic techniques depend upon the amplitude detection of the arrival of acoustic waves at a receiver. Such techniques are prone to errors generated by random noise occurring as a well logging instrument is moved through the borehole. Acoustic noise may be generated by the instrument body, or centralizers on the instrument body, scraping along the sides of the borehole as the tool is moved therethrough. Similarly, techniques involving pulsed acoustic transmitters for measuring shear waves or Stonely waves depends upon an elaborate interpretation of the waveform of the arriving wave at the receiver. Such interpretations are generally based on theoretical calculations done with simplified mathematical models of the earth formations in the vicinity of the borehole. If the simplified mathematical model proves to be in error, then the interpretation of the arriving waveform at the receiver may be in error, in its relationship to more complicated real life geometries and conditions than taken into account in the model.
It would be highly desirable to provide a method for measuring the travel time of various components of acoustic energy (compressional or primary wave, shear wave, Rayleigh wave, direct (fluid) wave, extentional wave, and Stonely wave) in earth formations in the vicinity of a well borehole which was not dependent upon a theoretical interpretation of an arriving acoustic pulse waveform. The system of the present invention provides a direct measurement of the travel time of all components of acoustic energy from a transmitter to a receiver in earth formations in the vicinity of a well borehole.